1. Field of the Invention
The present invention relates to an apparatus for supporting a vibrating object applicable to an apparatus for supporting a power unit (e.g., engine) of an automotive vehicle which effectively reduces various vibrations mainly caused by the power unit.
2. Description of the Prior Art
Conventional apparatus for supporting vibrating objects such as power units of automotive vehicles are exemplified by a Japanese Utility Model Application Unexamined Open No. Sho 57-142633 published on Sept. 7, 1982.
In the above-identified Japanese Utility Model Application, a liquid chamber formed of a cylindrically shaped elastic body is partitioned into a first and second liquid chambers through a partitioning wall. The partitioning wall has an orifice through which the first and second chambers are communicated and which is provided with an orifice valve for opening and closing the orifice. In addition, the orifice valve is connected electrically to a valve changeover apparatus so that an amount of flow through the orifice can be controlled.
However, there is a drawback in the conventional apparatus exemplified by the above-identified document.
That is to say, when the orifice valve is open, vibrations transmitted from the vibrating object are mainly absorbed in the elastic body constituting the liquid chambers. On the other hand, when the orifice valve is close, the vibrations are mainly damped by means of a flow resistance caused when liquid flows through the orifice which is not close.
Since, in the exemplified conventional apparatus, a dynamic damping effect of fluid has not completely been recognized, one of a plurality of vibrations having a particular frequency is accidentally reduced but no others of a plurality of vibrations having particular frequencies can effectively be reduced due to the set state of the apparatus.
A performance demanded for most effectively utilizing the apparatus for supporting the vibrating object of such a fluid sealing type will be described below before explaining the present invention.
The apparatus for supporting the vibrating object is required to have two functions simultaneously a vibration prevention function for preventing a transmission of minute vibrations caused by the vibrating object provided by the reduction of a dynamic spring constant of a fluid dynamic damper and a vibration damping function for damping large vibrations mainly caused by the vibrating object provided by an action of the fluid dynamic damper. The vibration prevention function required for the vibrating object supporting apparatus is achieved by a flow of an internal sealed fluid in the fluid chambers through the orifice along a vibration input direction so that the dynamic spring constant is reduced. The damping function also required for the apparatus is achieved by the fluid dynamic damper constituted by a fluid within the orifice as a fluid mass and elasticity caused by the flow of the sealed fluid between the chambers as a fluid spring. Therefore, it is most effective for the apparatus to use both, so called, vibration prevention and vibration damping functions.
In a case where the liquid chamber is partitioned into two chambers and the orifice provides means for communicating both chambers, the fluid dynamic damper is formed on the basis of a fluid within the orifice and a resonant frequency F of the fluid dynamic damper is expressed as follows: ##EQU1## wherein K denotes the fluid dynamic spring constant and M denotes an equivalent mass of a fluid within the orifice.
The resonant frequency F can thus be varied as the change in the fluid spring constant K or the change in the equivalent mass of the fluid within the orifice M. It is noted that the fluid equivalent mass M is determined by a real mass of fluid within the orifice and a relationship of cross sectional areas of the two fluid chambers to the cross sectional area of the orifice.
Hence, to reduce the plurality of vibrations by means of the fluid-sealed type vibrating object supporting apparatus, it is an essential performance required for the apparatus to enable the resonant frequency to be set at a different frequency through the change in the resonant frequency F and, in addition, to provide a fluidity for the fluid passing through the orifice having a predetermined fluid flow cross sectional area in order to exhibit a high vibration prevention function.